Many types of printing systems include one or more printheads that have arrays of marking elements that are controlled to make marks of particular sizes, colors, and the like in particular locations on the recording medium in order to print the desired image. In some types of printing systems, the array of marking elements extends across the width, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications so a carriage printing architecture is used instead.
In a carriage printing system (whether for desktop printers, large area plotters, and the like), the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped; printing is temporarily halted, and the recording medium is advanced. Then another swath is printed so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed on a printhead face along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction.
In some types of printers, such as inkjet printers, the face of the printhead die containing the array of marking element array is positioned near the recording medium in order to provide improved print quality. Close positioning of the printhead face to the recording medium keeps the printed dots close to their intended locations even for angularly misdirected jets.
In order to provide the capability of printing across the entire width of the recording medium, and also to permit space for the carriage to decelerate and stop before changing directions to print the next swath, typically the carriage moves the printhead beyond the side edges of the recording medium. Generally the position of the recording medium relative to the printhead face is fairly well controlled. However, occasionally a sheet of recording medium can have a dog-eared edge, a fold close to a corner that causes the corner to bend upwardly or downwardly. Also occasionally multiple sheets of recording medium can be inadvertently fed at the same time which sometimes causes paper jamming and folding of the recording medium in accordion fashion. In such situations, the close proximity of the printhead face to the position of the recording medium can result in the recording medium striking the face of the printhead as the carriage moves the printhead past the edge of the recording medium. For printhead faces made of a material that is fragile or brittle, such strikes can cause damage to the printhead, which requires replacement of the printhead.
U.S. Pat. No. 6,206,499 describes a head cover that overlaps the sides of the edges of the printhead die in order to prevent the nozzle plate from becoming damaged due to “paper stacking.” U.S. Pat. No. 7,862,147 describes providing an inclined surface that is positioned proximate to but not overlapping the edge of the printhead die, such that the recording media in the path of the oncoming printhead strikes the inclined surface and is deflected to avoid collisions with the fragile face of the printhead die. Although such a configuration reduces the occurrence of damaging collisions, it does not entirely eliminate the possibility of such collisions.
Consequently, a need exists for a way of detecting potential collisions between the recording medium and fragile portions of the printhead so that appropriate measures can be taken to avoid such collisions.